Polysaccharide SP-3 derived from Saposhnikovia divaricata (Turcz.) Schischk. enhances immunity by regulating intestinal targets and gut microbiota

• A homogeneous polysaccharide SP-3 was extracted from Saposhnikovia divaricata (Turcz.) Schischk. • The immunomodulatory effects of polysaccharide SP-3 were investigated for the first time. • Polysaccharide SP-3 could enhance immunity by regulating intestinal targets and gut microbiota. Saposhnikovia divaricata (Turcz.) Schischk. plays a significant role in immune regulation; however, studies on the immune regulatory activities of its constituent polysaccharides, especially homogeneous polysaccharides, are limited. This study aimed to investigate the immunomodulatory effects and mechanisms of a homogeneous S. divaricata polysaccharide, SP-3. Based on a preliminary exploration of the immunomodulatory activity of SP-3 in the Pseudomonas aeruginosa PA14-infected Caenorhabditis elegans , a mouse model of immunosuppression was constructed using cyclophosphamide (CTX) and the immunostimulatory effects and underlying mechanisms of SP-3 were investigated from the perspectives of intestinal mucosal immunity and the gut microbiota. SP-3 significantly extended lifespan, enhanced vitality, increased body fat content, and improved the reproductive capacity of P. aeruginosa PA14-infected C. elegans , which indicated that SP-3 can boost innate immunity. In the CTX-induced immunosuppressed mice, SP-3 protected the immune organs (spleen and liver), restored hematological parameters, including red blood cell and platelet counts, and enhanced systemic immunity. Mechanistically, SP-3 activated the homing pathway of immunoglobulin A (IgA) + plasma cells to promote IgA secretion in the small intestine and stimulated the TLR-NF-κB signaling pathway to enhance cytokine secretion by CD4 + T cells, thereby strengthening intestinal mucosal immunity. Additionally, analysis of the gut microbiota revealed that SP-3 improved microbial diversity, reduced the abundance of harmful bacteria ( Prevotellaceae, Alloprevotella, Rikenellaceae RC9 gut group ), and increased the abundance of beneficial bacteria ( Bacteroidaceae, Muribaculaceae, Lachnospiraceae, Lactobacillaceae, Ligilactobacillus ), effectively restoring CTX-induced intestinal dysbiosis. SP-3 acts as an immune enhancer to protect intestinal mucosal immunity by regulating intestinal targets and gut microbiota, thereby enhancing overall immunity.